The present application relates to radiation scanners, such as computed tomography (CT) scanners. It finds particular application with radiation scanners that comprise anti-scatters grids (also referred to as anti-scatter collimators) including one- and two-dimensional type grids.
CT and other radiography imaging systems are useful to provide information, or images, of targets (e.g., interior aspects) of an object under examination. Generally, the object is exposed to radiation, and an image is formed based upon the radiation absorbed by the targets, or rather an amount of radiation that is able to pass through the targets. Typically, highly dense targets absorb more radiation than less dense targets, and thus a target having a higher density, such as a bone or mass, for example, will be apparent when surrounded by less dense targets, such as fat tissue or muscle.
A radiation scanner typically comprises a detector array and a radiation source respectively mounted on diametrically opposing sides of an examination region within which the object under examination resides. In some scanners, such as three-dimensional imaging scanners (e.g., CT scanners), for example, the detector array and radiation source are mounted on opposing sides of a rotating gantry that forms a ring, or donut, around the object under examination. The rotating gantry (including the radiation source and/or detector array) is configured to be rotated in a circle situated within an x,y plane about an axis extending in the z-dimension (e.g., an “isocenter” and/or direction within which a focal spot of the scanner may “drift”) during an examination of the object (e.g., an object under examination, such as a suitcase, moves in the z-direction as it is moved into the scanner by a conveyor belt). As the rotating gantry is rotated, radiation can be intermittently or continuously emitted from a focal spot of the radiation source.
Radiation that traverses an object under examination is detected by one or more channels (also commonly referred to as pixels) of the detector array and respective signals are generated in response thereto. The signals are indicative of characteristics of the radiation that is detected by the respective channels, and thus is indicative of the attenuation of the object from a particular view, or projection.
In an ideal environment, the radiation that is detected by a channel of the detector array corresponds to attenuated radiation that strikes the channel on a straight axis from the focal spot of the radiation source. This type of radiation is commonly referred to as primary radiation. However, due to inevitable interactions with the object and/or radiation scanner, typically some of the radiation that is detected has deviated from the straight axis. Radiation that has deviated from the straight axis is commonly referred to as scattered radiation or secondary radiation. It will be appreciated that the detection of secondary radiation is undesirable because it can increase noise in a signal generated from the channel detecting the secondary radiation and/or it can reduce the quality of an image yielded from the signal.
In order to reduce the amount of secondary radiation that is detected by channels of the detector array, anti-scatter grids are commonly inserted between the examination region and the detector array. The anti-scatter grids are comprised of a plurality of anti-scatter plates configured to absorb secondary radiation and a plurality of transmission channels configured to allow primary radiation to pass through the grid and be detected by a channel of the detector array.
While the anti-scatter grids have proven effective for capturing secondary radiation, anti-scatter plates can impose shadows on the detector array. Such shadows can be detected by channels of the detector array and can create errors in the signals that are respectively generated by shadowed channels. If affected signals, or information derived from such signals, are not corrected (e.g., to take into account the shadowing), the shadowing may result in artifacts in an image yielded from the signals.
An air table(s) (also referred to as a calibration table) is commonly used to compensate for, or reduce the effects of, errors in the signals, including errors generated because of shadowing. During a calibration scan (e.g., a scan in which no object is present), signals generated by the respective channels are measured, and the measurements are stored in the air table(s). During examination scans (e.g., scans in which an object is present), the signals generated by respective channels are measured and compared with the measurement(s) stored in the air table(s) for the respective channels. The attenuation of radiation that is represented by the signal is determined by measuring the difference between the measurement(s) stored in the air table(s) and the measurement(s) generated during an examination scan. In this way, the portion of the signal indicative of the attenuation of radiation can be identified, for example.
The measurements acquired during the calibration scan and stored in the air table(s) are useful for compensating for errors in a signal as long as the errors are substantially static during the calibration scan and the examination scan(s). However, if the errors produced during a calibration scan and the errors produced during one or more examination scans differ, the difference between the measurement(s) stored in the air table(s) and the measurement(s) generated during the one or more examination scans may not be indicative of the attenuation of radiation. Stated differently, the measurements acquired during the calibration scan may not be accurate measurements for calibrating the respective channels (e.g., because a baseline value of the error in a signal has changed between the calibration scan and the one or more examination scans).
One common reason the signal can change between the calibration scan and one or more examination scans (or even change during a single examination scan) is due to dynamic shadowing. Dynamic shadowing occurs when the orientation of a shadow imposed on a channel by an anti-scatter plate changes between the calibration scan and the examination scan or between a first view of an examination scan and a second view of the same examination scan. It is commonly caused by focal spot motion (e.g., a change in the orientation of the focal spot with respect to the detector array). Such focal spot motion can be caused by thermal drift that occurs as the radiation source heats up and/or cools down during a scan, for example.
If the shadow imposed on a channel by an anti-scatter plate changes between the calibration scan and the examination scan, the measurement(s) stored in the air table(s) for the channel is typically adjusted before a correction mechanism uses the measurement(s) to isolate useful information in the signal from the errors. The adjusted measurement(s) reflects a (predicted) measurement(s) that would have been acquired during the calibration scan had the focal spot been oriented substantially similarly to its orientation during the examination scan.
To identify dynamic shadowing and determine how to accurately adjust the measurement(s) stored in the air table(s), the orientation of the focal spot is typically monitored, or rather calculated. Conventionally, a pin-hole camera placed near the radiation source has been configured to monitor the orientation (e.g., shape, position, etc.) of the fan, cone, or other shaped beam that impinges the detector array during the examination. The orientation of the focal spot can be determined based upon the orientation of the beam that is cast from the focal spot. A correction mechanism can then predict the orientation of shadows cast by the anti-scatter plates on respective channels and recalculate, or adjust, the air table measurement(s) for respective channels so that the attenuation caused by radiation can be determined from signals generated during an examination scan. In this way, the air table(s) comprises estimated measurements for respective channels that would have been produced during the calibration scan if the orientation of the focal spot during the calibration scan was substantially similar to the orientation of the focal spot during the examination scan.
While the use of a pin-hole camera to detect the orientation of the focal spot has proven successful in some applications, there are some drawbacks to such a technique. For example, it is typically difficult to make corrections to the signals, or information derived therefrom, on a view-by-view basis in real-time because of the complex calculations required to determine the orientation of the focal spot from a pin-hole camera image and to predict the orientation of the shadows produced based upon a determined focal spot orientation. Therefore, the pin-hole camera technique is typically limited to applications that use low resolution, slow-moving or stationary scanners. Additionally, making a determination about the orientation of the focal spot from a pin-hole camera image is not precise. Thus, corrections made to the signals, or resulting information, may be inaccurate. Such inaccuracies may result in artifacts being introduced.